Vehicle engine knock controls may retard spark timing relative to MBT (Minimum spark advance for Best Torque) in response to knock in order to abate the knock. However, drastically retarded spark timing may lead to unstable combustion without mitigating knock, for example at high IMEP (indicated mean effective pressure) and low engine speeds (e.g., less than 2000 rpm). Such conditions may be present in a direct injection engine including a boost system, such as a turbocharger, and may be exacerbated by high engine temperatures and/or using a low octane fuel.
One approach for controlling combustion instability during such knock control is described in WO 2008/029212. An amount of intake air is limited as a knock compensation value is large to a retarded side or as an intake air temperature value is high.
However, the inventors herein have recognized a problem with performance degradation resulting from such an approach. Specifically, as intake air is limited, engine output torque may correspondingly be reduced, thus reducing wheel torque to less than that desired by the driver. Furthermore, sudden transitions into and out automated control of the engine's intake air may result in further performance issues such as, for example, vehicle jerking.
Accordingly, methods are provided for controlling combustion stability in an engine driving a transmission. One example method comprises limiting airflow to the engine in response to a spark timing retarded beyond a spark retard threshold, the limiting airflow to the engine reducing engine torque output and compensating for the reduction in engine torque output by adjusting a transmission operating parameter.
In this way, it is possible to mitigate the effects of knock during high boost pressure and low engine speed conditions. Further, it is also possible to apply the above example method in combination with ramping in and ramping out automatic control of airflow to the engine as well as compensating for torque loss by changing transmission scheduling to mitigate vehicle performance degradation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.